The dumping of this sludge presents considerable problems. For instance, the readily water-soluble alkali-bound phosphates are washed out by rain and this will ultimately result in heavy eutrophication of the local surface water. At the same time, certain portions of the other sludge constituents are also washed out. In a wet phosphoric acid purification unit with an annular throughput of approximately 100,000 tons H.sub.3 PO.sub.4, the quantity of sludge obtained per year will be about 50,000 tons. According to the present state of engineering technology, there is no alternative but to dump this sludge. The cost of such a temporary dump is mainly determined by the land required, plastic film covering and/or lining and by the inspection requirements to ensure that the ground water is not pulluted. A final solution to the problem cannot be foreseen at present.
The aim of the invention is to utilize the waste sludge from wet phosphoric acid purification facilities without causing any problems. This aim is achieved by adding the waste sludge to the steelworks slag in controlled quantities, such that the P.sub.2 O.sub.5 content of the mixture amounts to 3 to 9%.